1. Field of the Invention
The present invention relates to a glass substrate of high strength for a magnetic disk and a process for its production.
2. Discussion of Background
Heretofore, aluminum alloy substrates have been used mainly as the substrates for magnetic disks to be used for magnetic disk memory devices. However, along with the demand for high density recording, an attention has been drawn to glass substrates which are materially hard as compared with aluminum alloy substrates and which are excellent in flatness and smoothness, and some of them have been practically used. However, glass substrates for magnetic disks which are made of glass as brittle material, have a problem that they are likely to break during their handling or during their use.
one of the factors governing the mechanical strength of a doughnut-shaped magnetic disk substrate made of glass, is flaws which are present on the surface of the inner peripheral edge of the substrate where the maximum tensile stress will form during the operation of the magnetic disk. Nevertheless, it is common that in the glass substrate for a magnetic disk, the surface roughness of the surfaces of the inner and outer peripheral edges (chamfer portions) is coarser than that of the finished main surface of the disk which is required to have extremely high levels of flatness and smoothness. The reason is simply that the inner and outer peripheral edge surfaces have no relation to magnetic recording, and they are curved surfaces which require costs for finishing treatment so high that the finishing treatment can not adequately be conducted.
On the other hand, in order to further improve the mechanical strength, it is common to reduce the depth of flaws on the inner and outer peripheral edge surfaces by conducting finish treatment of the inner and outer peripheral edge surfaces by means of abrasive particles having particle sizes finer than #500 mesh. Yet, there still remain considerably deep flaws on the inner and outer peripheral edge surfaces. To further improve the finish of the inner and outer peripheral edge surfaces, it is necessary to conduct multi-step processing using abrasive particles with their particle sizes stepwisely reduced. However, such multi-step treatment has had a problem of reducing the productivity to a large extent and substantially increasing the costs.
In the case of glass disks, it has been common, rather than improving the finish of the inner peripheral edge surface, to increase the strength by a chemical reinforcing method so-called an ion exchange reinforcing method, wherein glass is immersed in a molten potassium nitrate salt to ion exchange sodium ions on the glass surface with potassium ions of the molten potassium nitrate salt thereby to form a compression layer on the glass surface.
However, improvement of the strength by such chemical reinforcement is effective only for glass containing certain specific proportions of alkali metals such as sodium.
The depth of the surface compression stress layer introduced to the glass substrate surface by such a chemical reinforcing method and the level of the compression stress value, vary to some extent by such conditions as the temperature of the molten salt and the immersion time. However, they depend more largely on the composition of the glass itself rather than the temperature and the immersion time. Namely, in order to obtain a deep compression stress layer and to attain a high level of reinforcement, it is necessary to increase the proportions of alkali metal components such as sodium and lithium in the glass composition.
On the other hand, in a magnetic disk, an extremely thin metal or alloy magnetic layer is formed on the glass substrate. Therefore, there is a problem that as the alkali metal components such as sodium in the glass composition increase, such alkali metal components tend to substantially deteriorate the corrosion resistance of this magnetic layer. As a method for overcoming this problem, penetration of the alkali metal components into the magnetic layer can be prevented by forming an undercoat layer beneath this magnetic layer. However, in such a case, the undercoat layer is required to be sufficiently thick. Particularly in the case of glass containing a large amount of alkali metal components, the thickness of the undercoat layer is required to be sufficiently thick. Further, in the case of a doughnut-shaped glass substrate, it tends to be difficult to form such an undercoat layer with a sufficient thickness at the inner peripheral edge surface or at the outer peripheral edge surface, when it is formed by sputtering or vacuum deposition. Accordingly, corrosion of the magnetic layer is likely to form in the vicinity of the inner and outer peripheral edge surfaces. From the viewpoint of corrosion of the magnetic layer due to such alkali metal components, the durability of the magnetic disk is better as the proportion of alkali components in the substrate glass is small. On the other hand, if the proportion of alkali components is small, the depth of the surface compression stress layer of the glass substrate formed by ion exchange tends to be small, and it is likely to be less than the depth of flaws often present on the glass surface. Therefore, there have been drawbacks that the chemical reinforcing effect is small, and no adequate strength can be obtained.
As another characteristic of the glass substrate for a magnetic disk, the glass substrate is highly stiff and thus has an advantage that the plate thickness can be made thin. The trend for making the plate thickness thin has recently progressed rapidly, and it has already been started to use a glass substrate having a thickness of 0.381 mm, and such a thin glass substrate is believed to become the main commercial product.
When the plate thickness is thin, an excessively deep surface compression stress layer is likely to create a large tensile stress at the central portion in the thickness direction of the glass substrate and thus is likely to lead to deterioration of the strength.
On the other hand, etching treatment with hydrofluoric acid is widely known as a method for treating glass products in general. However, etching treatment with hydrofluoric acid has been considered to be not suitable for conventional glass substrates for magnetic disks and has not been practically employed. The reason is that excess etching treatment is likely to form undesirable high protrusions on the surface of the glass substrate for a magnetic disk. Namely, in a magnetic disk memory device, a magnetic head flies on the surface of a magnetic disk rotated at a high speed, at a height of from 250 to 500 .ANG. from the surface. Therefore, the presence of protrusions formed by excess etching is likely to cause head crush, which in turn is likely to destroy the entire recording surface of the magnetic disk. Accordingly, the height of abnormal protrusions must be controlled to be not higher than 250 .ANG. at the maximum.
It has been found that when the surface roughness of the inner and outer peripheral edge surfaces of a glass substrate for a magnetic disk obtained solely by conventional mechanical treatment by means of fixed and free abrasive particles, is measured by a three dimensional scanning electron microscope (SEM) (ESA-300, tradename: Erionix, manufactured by Erionix K.K., hereinafter sometimes referred to as SEM or Erionix) with a reference length of 240 .mu.m, the mean value of Ra at randomly selected four places is from 0.1 to 0.8 .mu.m, and the number of peaks is from 32 to 60, and the glass substrate for a magnetic disk having such surface roughness at the inner and outer peripheral edges has been found to have inadequate mechanical strength, as a test result.